Currently, intravenous tissue plasminogen activator administered within 3 hours of symptom onset is the only FDA-approved treatment for acute ischemic stroke. However, few patients qualify for this therapy, and there is an urgent need to identify alternative treatment strategies that extend the time-window with low hemorrhage risk. Endovascular mechanical embolectomy devices offer this promise. Moreover, the potential to identify an efficacious therapeutic approach may be optimized by employing patient-specific physiologic selection criteria (such as the presence of an MRI-defined ischemic penumbra). The MR RESCUE Trial will determine whether diffusion-perfusion MRI can identify patients who will benefit substantially from mechanical embolectomy, with the Concentric Retriever device for acute ischemic stroke 0-8 hours from onset. This study is a multi-center, randomized, controlled phase 2 trial of endovascular clot retrieval vs. conventional medical care in 120 patients with acute, large-vessel, anterior circulation ischemic stroke, stratified by MRI pattern (penumbral vs. non-penumbral). The primary hypothesis is that the presence of substantial ischemic penumbral tissue on MRI identifies, and the absence of treatment benefit (equivalency) in patients without penumbral regions, pre-treatment. Successful conduct of the trial will determine whether diffusion-perfusion MRI is an appropriate selection criterion for mechanical recanalization therapy for acute ischemic stroke. A positive trial will suggest substantial clinical benefit from embolectomy therapy in the group of patients with a penumbral MRI pattern, and permit informed design of definitive phase 3 trial(s) of endovascular embolectomy. A sub-study will utilize the thrombi extracted at endovascular mechanical embolectomy in trial patients to perform the first systematic anatomic, structural, and biochemical functional studies of platelet content, clot formation, and fibrinolysis in actual stroke-causing human thrombi.